Hi Just,
Though I would bring this bug to your attention, on completing a mission I ended up with a UFO Secured Score of -16777216.
Picture of the mission debrief https://dl.dropbox.com/u/31481417/mission_debrief.png
I though I had performed slightly better than that!
Before this error, I had just loaded the game from an autosave of before the mission start.
Let me know if you need any more details